Facebook is the headlines but it is hardly alone in digging up every last bit of information it can glean about you to make huge profits.
The king of this is Google, as I found out when I downloaded all of the data the two tech giants hold about me.
I wanted to find out just how much they knew. The answer, as you would expect, is everything I've ever done on their platforms.
But they also know about anything I've done on tools linked to their products. Facebook even has all the contacts and numbers in my phone.
The data I got from Google and Facebook showed me things I've long forgotten about are being stored by them, not for posterity – most of it is really boring - but for profit.
With it they can not only target me with clever advertising, but predict my future behaviour.
I'm not a big user of Facebook, but it had 68 megabytes of data stored about me – the equivalent of about 270 books (if a book is 200 pages).
That was dwarfed by Google – it had the equivalent of 31,311 books of data on me, more than seven gigabytes.
Imagine walking into a library and being surrounded by tens of thousands of books about all you've ever done on the internet.
On Facebook it is easy to get this data. I went on to settings and clicked 'Download a copy of your Facebook data.'
I got a notification about an hour later saying it was ready.
There were three folders in the file containing, amongst other things, all the contacts in my phone with names and numbers, all the events I've ever been invited to, every private message, public post and video I've sent on Facebook.
It also has the advertising topics it thinks I'm interest in – Journalist (my job), Germany (I used to live there) and Motor rallying (I once drove to Mongolia in a rally).
And then there are the thousands of photos I've shared on Facebook; things I have long forgotten; friends I've lost touch with, weekends away in Europe, student parties.
Google has more - more than 100 times as much data stored about me. I went to takeout.google.com to get it.
When I got an email the next day saying my data was ready, my computer crashed trying to download it.
The data came in three files. It had information from all the products connected to Google which I use – YouTube, Gmail, my calendar etc. complete with search history.
It knows I like watching England rugby victories, Mozart operas and rock music videos on YouTube. Big deal.
But put all this together with everything else it knows about it me and it can build a very complete profile.
All the books I've bought on Google, all the maps I've used, the websites I've bookmarked, a blog I once had, every document I've created on Google Drive, all transactions I've made using Google Pay, every email I've ever sent, every page I've visited.
But you sort of knew this already, right? Why does it matter what Google and Facebook know about you?
You're not up to anything criminal; your life is probably of no great excitement to Google. True. But it is very interesting to them because it is very profitable.
With this data anyone could build a profile about me with information I don't know about any more. Still not scared? You should be.
You gave it to them for free, because you wanted the ease of using their search engines and messenger services.
In return, you lost control of your personal information.
The Cambridge Analytica/Facebook scandal shows us this information may not be as secure as we think. It has been used by political parties and private firms to try to manipulate the way we behave – from what we buy to who we vote for.
Bear in mind companies like Facebook and Google can't standstill. Tech firms are going to try to find out more and more about you until they can better predict what you do. They are not evil, that is their business model and it is your fault, not their's, for giving them that information and power.
Google already predicts with great accuracy what you are going to do before you do it - from predicting what you will type into a search engine, to suggesting email replies. Smart fridges, meanwhile, can now order your weekly shop for you before you run out by keeping track of what is in your fridge.
The first organisation which knows you want a divorce, are looking for a new job or have an illness, before any of your nearest and dearest, is probably Google.
An algorithm could have worked out I was going to write this article before I told my editor and long before it was published.
It knows I'm a journalist at this newspaper, it knows I was requesting my data and it knows I was reading a thread on Twitter about this subject. That is fine for a subject like this, but what if I was writing in a country with a less tolerant government about a more controversial subject?
But short of going back to typewriters, encyclopaedias and a Nokia 3210 what can you do? Delete your browsing data and search history? No point – Google still stores it. Go offline? It's way too late for that.
Instead I'm weaning myself off Google and its products. I've downloaded a different browser; I've turned off the location on my phone, I've started using a search engine which doesn't track me and I've signed up to an encrypted email service.
Facebook and Google both say you are in control of what data you share with them and they use that data to make your life easier, but I'd rather not have every move I make monitored and stored. So after 12 years of giving everything away, I'm belatedly taking back control of my privacy.
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